How high would you have to go in the atmosphere for half of the mass of air to be below you?

Farmer a has a deep well and needs a lot of water for her animals and crops. her neighbor, farmer b, has a shallower well. he needs less water than farmer a because his farm is smaller. in years of drought, sometimes farmer b can't get any water from his wells. what can farmer b do to decrease the likelihood of his well running dry?